It has been known for several years that mammalian cells remove covalently bound mutagens and carcinogens from their DNA; however, few comparative quantitative studies have been attempted. We have begun such work by analyzing the removal of adducts of N acetoxy-2-aminofluorene in a variety of mammalian and human cells. To date, it is clear that somewhere between 20 and 50 percent of the carcinogen may be removed by G1 cells in culture, but the overall evaluation of the kinetics of removal are not yet available.